Prosthetic heart valves are used to replace damaged or diseased heart valves. In vertebrate animals, the heart is a hollow muscular organ having four pumping chambers: the left and right atria and the left and right ventricles, each provided with its own one-way outflow valve. The natural heart valves are identified as the aortic, mitral (or bicuspid), tricuspid and pulmonary valves. The valves of the heart separate chambers therein, and are each mounted in an annulus therebetween. The annuluses comprise dense fibrous rings attached either directly or indirectly to the atrial and ventricular muscle fibers. Prosthetic heart valves can be used to replace any of these naturally occurring valves, although repair or replacement of the aortic or mitral valves are most common because they reside in the left side of the heart where pressures are the greatest. In a valve replacement operation, the damaged leaflets are excised and the annulus sculpted to receive a replacement valve.
The four valves separate each ventricle from its associated atrium, or from the ascending aorta (left ventricle) or pulmonary artery (right ventricle). After the valve excision, the annulus generally comprises a ledge extending into and defining the orifice between the respective chambers. Prosthetic valves may attach on the upstream or downstream sides of the annulus ledge, but outside of the ventricles to avoid interfering with the large contractions therein. Thus, for example, in the left ventricle a prosthetic valve is positioned on the inflow side of the mitral valve annulus (in the left atrium), or on the outflow side of the aortic valve annulus (in the ascending aorta).
Two primary types of heart valve replacements or prostheses are known. One is a mechanical-type heart valve that uses a ball and cage arrangement or a pivoting mechanical closure to provide unidirectional blood flow. The other is a tissue-type or “bioprosthetic” valve which is constructed with natural-tissue valve leaflets which function much like a natural human heart valve, imitating the natural action of the flexible heart valve leaflets which seal against each other to ensure the one-way blood flow.
Prosthetic tissue valves typically comprise a stent having a rigid, annular ring portion and a plurality of upstanding commissures to which an intact xenograft valve or separate leaflets of, for example, bovine pericardium are attached. Fabric covers a majority of the structure and a suture-permeable sewing ring is provided around the periphery for attaching to the natural annulus. Because of the rigidity of the material used in the stent and/or wireform, conventional valves have a diameter that is minimally affected by the natural motion of the heart orifice. In the aortic position, the prosthetic valve commissures extend axially in the downstream direction a spaced distance from the walls of the downstream aortic wall. Movement of the aortic wall or sinuses does not directly affect movement of the cantilevered commissures, though fluid flow and pressures generated by movement of the walls ultimately does cause the commissures to dynamically flex to some extent (i.e., they are cantilevered downstream in the aorta). Because of the inherent rigidity in conventional heart valves, the natural dilatation of the annulus is restricted, imposing an artificial narrowing of the orifice, and increasing the pressure drop therethrough.
One alternative to more rigid prosthetic aortic heart valves is seen in U.S. Pat. No. 6,558,418 to Carpentier, et al., and U.S. Pat. No. 6,736,845 to Marquez, et al., the disclosures of which are expressly incorporated by reference herein. Carpentier, et al. and Marquez, et al. disclose a flexible heart valve especially suitable for implant in the aortic position. A single or multi-element stent includes commissures constructed so that the cusps are pivotably or flexibly coupled together to permit relative movement therebetween. An undulating suture-permeable connecting band attached to the valve follows the cusps and commissures and extends outwardly such that the valve may be connected to the natural tissue using conventional techniques. The connecting band may be cloth-covered silicon and provides support to the stent and to the outer side of the valve at the commissures while still permitting the cusps to flex with respect to one another.
Despite serious attempts to create a flexible heart valve that responds to the natural motions of the annulus and downstream vessel walls, even more flexible valves are desirable.